The present invention is directed to a high-power, integrated AC switch module particularly for use in AC-AC power converters, solid state contactors and AC bus controllers with improved fault protection capability.
Primary electric power on commercial and military aircraft is provided by 400 Hz constant-frequency (CF) 3-phase 115V AC power. These power systems utilize a complex hybrid unit called the “Integrated Drive Generator” (IDG) to convert the mechanical power at variable-speed rotation of each aircraft engine into a constant frequency power at 400 Hz. In an IDG unit, the variable speed input is first converted to a regulated constant speed by a built-in mechanical/hydraulic mechanism. The constant frequency power is then generated by an alternator coupled on the shaft of the IDG with the constant rotating speed. IDG based systems have low efficiency, are costly, and are maintenance intensive.
Future aircraft electrical power systems will employ a variable frequency (VF) power system or a hybrid system configuration (VF+CF) for ever increasing electrical power requirements for on-board AC loads and AC motor controls. Replacing the conventional CF AC with VF power distribution enables reducing system weight, volume, and cost, while improving system overall efficiency and reliability. VF AC power distribution and AC-AC regenerative converters or drives also may find application in future hybrid electrical vehicles, such as electrical compact vehicles and hybrid-electrical buses, where high power ratings and regenerative operation are required.
However, in such VF or VF-plus-CF power systems, various solid state power converters must be used to properly and efficiently control the AC motors and other AC loads. Among the various converter topologies, VF-input AC-AC power conversion based on four-quadrant (bi-directional) AC power modules offers many advantages, including:                elimination of temperature and weather sensitive passive power components, such as DC-link capacitors. This increases system service life and reliability, reduces regular maintenance requirements, and elevates designed operating temperature, thus reducing cooling requirements.        large percentage reduction of weight and volume through the elimination of large reactive power components, such as DC-link electrolytic capacitors and inductors.        higher power density.        inherent bi-directional power control.        improved energy conversion efficiency at all rotating speeds and load conditions of electrical machines.        
However, switching loss associated with the high-frequency switching for pulse-width modulation contributes significantly to AC-AC converter power loss. During commutation, the AC switching devices, which are building elements of AC-AC converters, see additional narrowly-shaped current pulses in superposition to the regular load current. Those narrowly-shaped current pulses are due to the non-ideal power diode's recovery process. This potentially reduces the effectiveness of device's silicon utilization.
Previous bi-directional power modules were based on the use of silicon isolated gate bipolar transistors (IGBTs), and fast-recovery silicon diodes. IGBTs and freewheeling diodes at chip level were connected and integrated into individual AC switches. Each AC switch conducted current in either direction and blocked voltage in both directions, thus making a four-quadrant bi-directional power switch. The current and voltage ratings for the diodes equaled that of the IGBT in the AC switch configuration, since the same current passed through the IGBT and diode in each direction. However, silicon power diodes possess relatively large transient reverse recovery current, even though fast-recovery diodes are used. In addition to contributing to converter switching losses, the transient reverse current also passes through the IGBT devices, which are in a serial connection in the AC power switch, such that the current margins of the devices are undesirably reduced.
Better switching performance can be obtained by using wide band-gap power devices, such as Silicon Carbide (SiC) or Silicon Nitride (SiN) devices. However, today's SiC transistor devices are in early lab development and their ratings are limited to small current levels, i.e., less than about 10 amps, at 1200 volts and 600 volts. SiN based power devices are even less developed. On the other hand, SiC diodes have high power density and desirable reverse recovery characteristics, but have low current ratings.
Another problem addressed by the present invention is heat dissipation of SiC diodes in a single chip design such as in a fully sealed environment inside a power module. This is a problem because the working space is congested, and heat flux density of SiC diodes is several times higher than that of silicon power diodes, as the size of SiC diodes is much smaller at an equivalent current rating.